Rein Silberberg

Neutron Generated Single-Event Upsets in the Atmosphere

Heavy cosmic ray nuclei are mostly attenuated with a shielding of 50 g/cm2 atmospheric gas. However, the shielding acts as a generator of neutrons, evaporated or knocked out of nuclei. These neutrons generate highly ionizing nuclear recoils that produce single-event upsets in microelectronic components. To attenuate the secondary neutron flux over 300 g/cm2 of atmospheric material is required. The numerous slow protons from nuclear interactions in shielding will also genetrate upsets in sensitive components, which have a low critical charge. At altitudes below 65,000 feet, most single-event upsets are due to these secondary particles. The upset rates due to neutrons and slow secondary protons from cosmic ray, solar flare particle, and trapped radiation particle interactions are presented as a function of the critical charge.

Comparison of distributed reacceleration and leaky-box models of cosmic-ray abundances (Z = 3-28)

A large collection of elemental and isotopic cosmic-ray data has been analyzed using the leaky-box transport model with and without reacceleration in the interstellar medium. Abundances of isotopes and elements with charges Z = 3-28 and energies E = 10 MeV/nucleon-1 TeV/nucleon were explored. Our results demonstrate that reacceleration models make detailed and accurate predictions with the same number of parameters or fewer as standard leaky-box models. Ad hoc fitting parameters in the standard model are replaced by astrophysically significant reacceleration parameters. Distributed reacceleration models explain the peak in secondary-to-primary ratios around 1 GeV/nucleon. They diminish the discrepancy between rigidity-dependent leakage and energy-independent anisotropy. They also offer the possibility of understanding isotopic anomalies at low energy.

Electron capture decay of cosmic rays

Cosmic-ray nuclei are close to fully ionized during their passage through the Galaxy. Electron capture decay is rare among these nuclides because most do not have bound electrons. Under certain conditions, specifically low energies and/or high charges, electron capture becomes an essential factor in determining cosmic-ray composition. In this paper we discuss the general nature of electron capture decay in cosmic rays and describe specific measurements which can reveal the existence of electron capture decay, and energy and density-dependent processes in the interstellar medium.

Matrix Methods of Cosmic Ray Propagation

Matrix methods of cosmic ray propagation involve reduction of the propagation equations to a set of coupled linear differential equations. This reduction results in an elegant separation of local propagation effects, such as nuclear fragmentation, from global effects such as pathlength distribution. The matrix equations are solved by simple methods. We show how the reduction to linear form is made for ionization loss and solar modulation. In addition, we show how to calculate propagation errors.

Cosmic-Ray Heavy Ions at and above 40,000 Feet

The flux and LET-spectra of heavy cosmic ray nuclei and their secondary progeny have been calculated at aircraft flight altitudes. The associated frequency of single event upsets is presented and compared with neutron-induced events.

Galactic Cosmic Radiation Doses to Astronauts Outside the Magnetosphere

The dose and dose equivalent from galactic cosmic radiation outside the magnetosphere have been computed. Each of the principal radiation components were considered. These include primary cosmic rays, spallation fragments of the heavy ions, and secondary products (protons, neutrons, alphas, and recoil nuclei) from interactions in tissue. COnventional quality factors were used in converting from dose to dose equivalent.

On the abundances of ultraheavy cosmic rays

Recent data from the HEAO 3 and Ariel 6 satellites on elemental abundances of ultraheavy cosmic rays (33< or =Z< or =83) are analyzed using a new propagation code. General agreement with earlier analyses is observed. Evidence for a breakdown of the correlation between ionization potentials and the solar system/cosmic-ray source abundance ratio is presented. We find that the best fit to experimental data (approx.5 GeV per nucleon) occurs when propagation is calculated at lower energies (approx.1 GeV per nucleon). This is interpreted as evidence for distributed acceleration of cosmic rays. Additional effects, including ionization loss, altered path-length distributions, and r-process enhancement, are considered.

A comparison of neutron-induced SEU rates in Si and GaAs devices

The single-event-upset rates due to neutron-induced nuclear recoils have been calculated for Si and GaAs components using the HETC and MCNP codes and the ENDF data base for (n, p) and (n, alpha) reactions. For the same critical charge and sensitive volume, the upset rate in Si exceeds that of GaAs by a factor of about 1.7, mainly because more energy is transferred in neutron interactions with lighter Si nuclei. The upset rates due to neutrons are presented as functions of critical charge and atmospheric altitude. Upsets induced by cosmic-ray nuclei, secondary protons and neutrons are compared.

Improvement of Calculations of Cross Sections and Cosmic-Ray Propagation

The cosmic-ray experiments on the HEAO-3 satellite provide unprecedented data, both in terms of the number of events and in elemental resolution. The two experiments together have yielded measurements all the way from lithium to uranium. Our partial cross section calculations have a standard deviation of ~ 30 per cent; higher precision is desirable now for the analysis of the high-quality experimental results on relative abundances and energy spectra of elements. We have collected data on cross section measurements of the last five years, in order to explore the existence of systematic deviations in our equations and parameters for calculating cross sections. During the first phase of this work, we have emphasized the development of an empirical formula for total inelastic cross sections. The results of this investigation (both on total and partial inelastic cross sections) will be presented, and the effects on the calculation of cosmic-ray abundances will be discussed. In particular, the discrepancy between earlier propagation calculations and measurements of the secondary/primary ratio (6l ≤ Z ≤ 75)/(76 ≤ Z ≤ 83) is appreciably reduced.

Cosmic Ray Transport in the Atmosphere: Dose and Let-Distributions in Materials

Heavy nuclei in cosmic rays generate single event upsets in microelectronic components. We have calculated the LET-distributions and the fluxes of the various cosmic ray nuclei for elements 1 ¿ Z ¿ 28. These radiation transport calculations extend to altitudes of 55,000 feet, i.e. to aircraft flight altitudes. The calculations agree well with the limited experimental data available. The results of calculation of the upset rate vs. the critical charge are presented.